Relative angular displacement, expressed in degrees, refers to the integral of the rotation speed of the said mobile as a function of time. This type of equipment is used notably for aeronautical applications.
The gyrolaser, developed three decades ago, is widely marketed and used nowadays. Its operating principle is based on the Sagnac effect, which induces a frequency difference Δνs between the two so-called counter-propagating optical emission modes, propagating in the opposite direction, of a bidirectional ring laser cavity to which a rotational movement is imparted. Classically, the frequency difference Δνs is given by the following equation:Δνs=4Aω/λL  (A)where L and A are respectively the wavelength and area of the cavity; λ is the laser emission wavelength without the Sagnac effect; ω is the rotation speed of the assembly.
The frequency difference Δνs between the two optical modes is measured by spectral analysis of the interference of the two emitted beams. It makes it possible to ascertain the value of ω with very high precision.
The condition for observation of the interference is stability and relative equality of the intensities emitted in the two directions. It is not a priori an easy thing to obtain owing to the phenomenon of competition between modes, which means that one of the two counter-propagating modes may have a tendency to monopolize the available gain, at the cost of the other mode.
This problem is typically resolved in solid-state gyrolasers by introducing into the cavity optical losses which depend on the propagation direction of the optical mode and its intensity. The principle is to modulate these losses by a feedback device, as a function of the intensity difference between the two emitted modes, in order to favour the weaker mode at the cost of the other so as to constantly maintain equilibrium between the two counter-propagating modes.
Patent Application FR0303645, filed by the Applicant, has proposed a stabilizer device for a solid-state gyrolaser, which consists of a feedback system imposing optical losses which depend on the propagation direction, while being based on the combination of three physical effects: reciprocal rotation, non-reciprocal rotation and polarization.
FIG. 1 represents the overall diagram of a conventional gyrolaser 100. It comprises a ring cavity 1 consisting of at least three mirrors 11, 12 and 13, a solid-state amplifier medium 19 and a device 30 for stabilizing the intensities, comprising a polarizing element 71, a reciprocal effect device 7 acting on the polarization state of the counter-propagating modes and a non-reciprocal effect device 8 also acting on the polarization state of the counter-propagating modes, at least one of the effects of the said devices being controllable. The assemblies 71, 7 and 8 are arranged on the paths of the counter-propagating beams. The gyrolaser, having a solid-state amplifier medium 19, is referred to as a solid-state gyrolaser.
There is a non-reciprocal optical effect in an optical component when, the light having an initial polarization state, the polarization state of the light is different from this initial state after a return journey in the said component. Thus, the same beam travelling in the opposite direction in a non-reciprocal optical rotator will experience a rotation of its polarization direction in the same sense. There is a reciprocal optical effect in an optical component when, the light having an initial polarization state, the polarization state of the light is identical to this initial state after a return journey in the said component.
At the exit of the cavity, the two optical modes 5 and 6 are superposed by a superposition means 44 in order to give a useful signal Su, constituting interference of the two counter-propagating modes whose frequency Δνmes is equal to the frequency difference between the two optical modes. The gyrolaser also comprises a means for determining a rotation measurement of the gyrolaser 3. Measuring the rotation of the gyrolaser is intended to mean measuring a quantity which represents the rotation of the gyrolaser. This is, for example, a measurement of the rotation speed Ω of the gyrolaser (expressed in rad·s−1) or a measurement of the relative angular displacement IΩ of the gyrolaser (corresponding to the integral of the rotation speed as a function of time) expressed in rad. This measurement means 3 comprises, for example, a photodiode adapted to measure the frequency difference between the two modes. The rotation speed Ω of the gyrolaser 100 is calculated from the measured frequency difference Δνmes on the basis of Equation A.
As a variant, the means 3 comprises a means for measuring the speed and the rotation direction from the number of fringes (associated with the frequency difference between the two modes) passing in one direction and in the other. This means 3 has, for example, two photodiodes arranged in quadrature relative to the fringes of the interference signal.
The angular displacement is calculated by integrating the measurement of the rotation speed over time.
When calculating the rotation measurement of the gyrolaser in the prior art, it is assumed that the frequency difference between the two counter-propagating modes is due exclusively to the rotation of the gyrolaser (Sagnac effect).
A fraction of these beams 5 and 6 is sampled by means of two semi-reflective plates 43 and sent on to two photodetectors 42. The intensity of the beams 5 and 6 may also be measured directly at the exit of the cavity, by using a second output coupler (the first being used in this case only to measure the interference signal).
The signals delivered by these two photodetectors represent the luminous intensity of the two counter-propagating modes 5 and 6. The signals are sent to a feedback electronics module 4, which drives the variable effect device as a function of the intensity of the received signals (dashed arrows on the diagram). In conjunction with the polarizing device 71, this will result in variations of the polarization states of the two counter-propagating beams. These polarization state variations will thus lead to different optical losses in the counter-propagating optical modes 5 and 6. If one of the beams has a greater luminous intensity than the other, its intensity will be attenuated more so as to return the output beams to the same intensity level. The bidirectional regime is thus stabilized in intensity, and stable and balanced bidirectional emission is obtained.
A drawback of conventional gyrolasers is the existence of a bias in the rotation measurement of the gyrolaser, this bias being induced by the device for stabilizing the intensities.